launcher will not relaunch apps with rotated second monitor

In trusty, up to date as of 7th April...

Symptom:

Launching an app by clicking its icon on the launcher, and then closing the app, will not allow you to launch it again by clicking its icon on the launcher.

Steps to reproduce:

1) Connect two monitors
2) In Screen Display settings, position the primary monitor to the right of the secondary
3) Set the secondary monitor (now on the left) to rotate anticlockwise
4) Set the launcher to be on the primary monitor
5) Enable sticky edges
6) Apply and accept the configuration
7) Launch any application from the launcher, then close the application
8) Click the application on the launcher to try to launch it again, and nothing happens.

If you don't rotate the secondary monitor, everything works fine. If you have the secondary monitor on the right instead of the left, everything works fine.